1. Field of the Invention
The present invention relates to a reverse conducting gate turn-off thyristor and, more particularly, to a structure of an isolating zone between thyristor and diode portions of the reverse conducting gate turn-off thyristor.
2. Description of the Background Art
A reverse conducting gate turn-off thyristor is an integrated device which includes on a wafer a gate turn-off thyristor and a free wheel diode connected antiparallel to the gate turn-off thyristor. Usually, on a wafer, a resistance isolates a thyristor portion serving as a gate turn-off thyristor from a diode portion serving as a feedback diode. This device miniaturized a device such as an inverter incorporating a gate turn-off thyristor.
FIG. 1A is a top plan view showing a half of the top surface of a conventional reverse conducting gate turn-off thyristor of compression bonded type. FIG. 1B is a sectional side view showing the structure taken along the line I--I in FIG. 1A. Reference symbols X, Y and Z indicate a thyristor portion, a diode portion and an isolating zone, respectively.
Referring to FIG. 1B, a p type base layer 2 and an n.sup.+ type layer 3 are formed on the upper and lower surfaces of an n type base layer 1, respectively. In a thyristor portion X, a plurality of p type emitter regions 4 are selectively formed within the n.sup.+ type layer 3, and a plurality of n type emitter regions 5 are selectively formed upon the p type base layer 2. In an isolating zone Z, a trench 6 is provided in the p type base layer 2 by means of selective etching or the like. A remaining part under the trench 6 of the p type base layer 2 of a low concentration serves as an isolation resistance region 7. As can be seen from FIG. 1A, the trench 6, and therefore the isolation resistance region 7 surround the thyristor portion X to isolate the thyristor portion X form a diode portion Y.
Cathode electrodes 8 are provided on the n type emitter region 5 and the p type base layer 2 in the diode portion Y. A gate electrode 9 is provided on the p type base layer 2 in the thyristor portion X. A portion of the gate electrode 9 in the center serves as a gate collecting electrode, which is denoted by a reference numeral 9a. Cathode and gate electrodes 8 and 9 are insulated by an insulation film 10. An anode electrode 11 is provided on the back surface of the n.sup.+ type layer 3 and p type emitter regions 4. The anode electrode 11 short-circuits the n.sup.+ type layer 3 and the p type emitter regions 4 to form a shorted emitter structure, whereby turning-off capability can be increased.
FIG. 2 is a circuit diagram illustrating an equivalent circuit in the above reverse conducting gate turn-off thyristor. A gate turn-off thyristor 12 consists of the p type emitter regions 4, n type base layer 1, p type base layer 2 and n type emitter regions 5 in the thyristor portion X of FIG. 1B. A free wheel diode 13, which is in antiparallel connection with the gate turn-off thyristor 12, consists of the n type base layer 1 and p type base layer 2 in the diode portion Y of FIG. 1B. An isolation resistance R is formed by a sheet resistivity in the isolation resistance region 7 of FIG. 1B. The isolation resistance R is provided between the gate and cathode of the gate turn-off thyristor 12 in the equivalent circuit.
The above reverse conducting gate turn-off thyristor does not have a shorted emitter structure with respect to the n type emitter region 5. Accordingly, when this conventional reverse conducting gate turn-off thyristor is turned off, the PN junction defined by the n type emitter region 5 and the p type base layer 2 must remains reverse-biased in order to inhibit electrons from flowing from the n type emitter region 5 into the p type base layer 2. Hence, to turn off the thyristor, the cathode and gate electrodes 8 and 9 are reverse-biased. At this time, reactive current flows through the isolation resistance R, which increase a burden on a gate driving circuit not shown. Therefore, it is desirable that the isolation resistance R has a value as large as possible.
A value of the isolation resistance R is uniquely determined by the resistivity of the isolation resistance region 7 and the width of the isolating zone Z. The value of the isolation resistance R of the annular isolating zone Z shown in FIG. 1A is given by the following equation; EQU R=.rho..sub.PB .times.1n (r.sub.2 /r.sub.1)
where .rho..sub.PB is a resistivity of the isolation resistance region 7, i.e., the p type base layer 2, r.sub.1 is an inner diameter of the isolating zone Z, and r.sub.2 is an outer diameter thereof. The resistivity .rho..sub.PB of the p type base layer 2 is restricted in increase in view of a forward blocking voltage of the device. Specifically, assuming that the impurity profile of the p type base layer 2 is constant, voltage applied between the anode and cathode permits a depletion layer to extend uniformly into the p type base layer 2 from the PN junction defined by the n type base layer 1 and the p type base layer 2, while the thyristor is turned off. If the depletion layer reaches up to the surface of the isolation resistance region 7, it is feared that the breakdown of the device is caused. To prevent this, it is necessary to make the resistivity .rho..sub.PB of the p type base layer 2 small enough. On the other hand, shortening the inner diameter r.sub.1 of the isolating zone Z is restricted in view of current capacity. In other words, the thyristor portion X must have a sufficient area to make a desired current capacity available, so that the inner diameter r.sub.1 of the isolating zone Z can not be made smaller than the value corresponding to this sufficient area of the thyristor portion X.
Thus, in order to increase the isolation resistance R, it is necessary to make the outer diameter r.sub.2 of the isolating zone Z larger, that is, to broaden its width (r.sub.2 -r.sub.1). Since the isolating zone Z functions inactively in the operation of the reverse conducting gate turn-off thyristor, broadening the width (r.sub.2 -r.sub.1) of the isolating zone Z to increase its area causes the efficiency of utilization of a wafer surface in the reverse conducting gate turn-off thyristor to decrease.
Further, in order to make the reverse conducting gate turn-off thyristor shown in FIGS. 1A and 1B have a large capacity, current capacity in the thyristor portion X and diode portion Y must be increased. To increase current capacity in the thyristor portion X, the inner diameter r.sub.1 of the isolating zone Z must be increased. Thus, if the value of the isolation resistance R and therefore (r.sub.2 /r.sub.1) are kept constant, the width of the isolating zone (r.sub.2 -r.sub.1) must further be increased. As a result, with the reverse conducting gate turn-off thyristor having a greater capacity, the efficiency of utilization of the wafer surface will be further reduced.
The reverse conducting gate turn-off thyristor shown in FIGS. 1A and 1B has to drain charges accumulated in the p type base layer 2 out through the gate electrode 9 by reverse-biasing between the cathode and gate to block the anode current. Reverse gate current flowing in the gate at this time is determined with the value of the anode current and the turn-off gain of the thyristor. The turn-off gain is usually 3 to 5, and accordingly, 1/3 to 1/5 of the main current has to be drained outside through the gate 9. Thus, gate turn off thyristors need a gate conducting capability much greater than ordinal thyristors. The reverse conducting gate turn-off thyristor shown in FIGS. 1A and 1B is provided with a gate collecting electrode 9a which is in contact with a gate external electrode not shown, for flowing such large current. The area of the gate collecting electrode 9a has to be increased to increase the current capacity of the device, so that the efficiency of utilization of the wafer surface is reduced due to the area required for the gate collecting electrode 9 a.
As hereinbefore described, the conventional reverse conducting gate turn-off thyristor has a disadvantage that the efficiency of utilization of the wafer surface is decreased when the resistance value of the isolation resistance R isolating the thyristor portion X from the diode portion Y is increased, or when the current capacity of the device is increased.